The present invention relates to a brushless motor which controls the rotation of a rotor on a single-phase basis.
FIGS. 1 to 3 show an arrangement of a prior art brushless motor of the type referred to. More specifically, FIG. 1 shows a cross-sectional view of the prior art brushless motor showing an array of parts in the motor, FIG. 2 is an exploded perspective view of the parts of the motor, and FIG. 3 is a block diagram of the brushless motor. In the drawings, a casing 1 comprises a lower casing 1A and an upper casing 1B. Rotatably supported within the casing 1 is a rotor 2. Mounted on the bottom of the rotor 2 are permanent magnets 3 and 4 at positions symmetrical with respect to the center of the rotor 2. A coil 5 is mounted on the lower casing 1A. When a current is passed through the coil 5, this causes a magnetic field to be generated so that the permanent magnets 3 and 4 are attracted to each other and therefore the rotor 2 is turned. A magnet 6 is disposed to the lower casing 1A to position the rotor 2 at its initial position between the permanent magnets 3 and 4. When a power source is turned off, the magnet 6 for positioning the rotor 2 at its initial position has the same polarity as that of the permanent magnets 3 and 4, which results in that the rotor 2 stops at its balanced position because the magnets repel each other. A magnetic sensor 7 such as a Hall effect element, is mounted together with another electronic element 8 on a substrate 9 which in turn is disposed on the lower casing 1A. In more detail, the magnetic sensor 7 is located at such a position as to be subjected to the influence of magnetic force of the permanent magnets 3 and 4 of the rotor 2. The substrate 9 having such parts mounted thereon is disposed on the lower casing 1A and then molded with resin material 10. Referring to FIG. 3, between the coil 5 and the power source, a switching element 11 such as a transistor is inserted. The switching element 11 is connected with a current limiting element 12 such as a resistor which in turn is connected with an output terminal of the magnetic sensor 7. That is, the magnetic sensor 7 detects the positions of the permanent magnets 3 and 4 and generates an output signal that is sent to the switching element 11. The switching element 11 controls supply of a current to the coil 5 on the basis of the output signal received from the magnetic sensor 7. In FIG. 3, reference numeral 13 denotes a constant-voltage diode, 14 a resistor.
In the operation of such an arrangement, when the permanent magnet 3 is positioned above the magnetic sensor 7, the magnetic sensor 7 generates an on-state output and thus the switching element 11 is turned on so that a current flows through the coil 5, thus resulting in that a torque for attracting the permanent magnet 4 is generated in the coil 5 and the rotor 2 is rotated. As the permanent magnet 4 moves toward to the vicinity of the coil 5, the permanent magnet 3 moves away from the vicinity of the magnetic sensor 7, whereby the magnetic sensor 7 generates an off-state output and thus the switching element 11 is turned off. This causes the current so far flowing through the coil 5 to be stopped so that the attractive force between the permanent magnet 4 and the coil 5 disappears and the rotor 2 is continuously coasting. Thereafter, the permanent magnets 3 and 4 become opposite in state to each other and operatively move in a similar manner to what mentioned above.
A force resisting the movement of the rotor 2 trying to continuously coast exists as a repulsive force between the magnet 6 for positioning of the initial position of the rotor 2 and the permanent magnets 3 and 4 of the rotor 2, and the magnetic sensor 7 has a hysteresis width (which will be referred to as the BW, hereinafter) between a detection level (which will be referred to as the BH-L, hereinafter) changing from its detection off to detection on and a detection level (which will be referred to as the BL-H, hereinafter) changing from its detection on to detection off. For this reason, in the case where the rotor 2 loses an attractive force caused by the coil 5 at the BL-H and is subjected to the influence of the repulsive force of the magnet 6 for positioning of the initial position of the rotor 2 to thereby start to reversely rotate, the rotor 2 returns to the BH-L and again normally rotates under the influence of the attractive force of the coil 5, which is repeated several times. After this, acceleration obtained during a period between the BL-L and BL-H causes the rotor 2 to overcome the repulsive force of the magnet 6 for positioning of the initial position of the rotor 2.
Such a prior art brushless motor, however, has had a problem that variations in the BW of the magnetic sensor cause the motor not to be able to rotate under the worst conditions (when the BW is narrow).
The current-level technique of suppressing variations in the BW of the magnetic sensor has its limit and thus it has been impossible to solve the problem.